In recent years, a high pressure acid leaching method (HPAL method) using sulfuric acid has attracted attention as a wet smelting process for recovering nickel and cobalt from nickel oxide ore containing iron as a main component and nickel at from 1% by mass to 2% by mass. This method does not include dry treatment steps such as drying and sintering steps but includes a consistently wet step unlike general conventional nickel oxide ore smelting methods, and it is thus advantageous in terms of energy and cost.
Specifically, the nickel smelting method using this high pressure acid leaching method includes, for example, a step of preparing an ore slurry by slurrying nickel oxide ore (ore slurry preparation step), a step of obtaining leached slurry by adding sulfuric acid to the ore slurry and subjecting the ore slurry to a leaching treatment at a high temperature of from 220° C. to 280° C. and a high pressure in a pressure leaching reactor (hereinafter, an “autoclave” will be described as a specific example) to leach out the nickel and cobalt in the ore (leaching step), a step of separating the leached slurry into the leach residue and the leachate containing nickel and cobalt by solid-liquid separation (solid-liquid separation step), a step of neutralizing and separating impurity elements such as iron by adjusting the pH of the leachate containing the impurity elements together with nickel and cobalt to from 3 to 4 (neutralization step), and a step of supplying a sulfidizing agent such as hydrogen sulfide gas to the leachate after being neutralized and separated and recovering nickel-cobalt mixed sulfide (sulfidization step).
Such a high pressure acid leaching method is significantly greatly advantageous since nickel and cobalt can be selectively leached while iron of the main impurity is immobilized in the leach residue in the form of hematite (Fe2O3) by controlling the oxidation-reduction potential and temperature of the leachate in the autoclave in the leaching step.
Here, in the leaching step, sulfuric acid is added to a compartment 1a of an autoclave 1 through two sulfuric acid addition pipes (lines) 11A and 11B, for example, as illustrated in FIG. 1. Nickel and cobalt and further impurities and the like are leached out from the ore while the ore slurry to which sulfuric acid is added sequentially moves from the compartment 1a to a compartment 1g which have a stirrer.
As a pump (sulfuric acid supply pump) for supplying sulfuric acid, a diaphragm pump, which is a type of positive displacement pump, is frequently used. Conventionally, the sulfuric acid supplied from a sulfuric acid supply pump P (PA, PB) is simultaneously supplied to the autoclave 1 via two sulfuric acid addition pipes LA and LB, for example, as illustrated in FIG. 8. This sulfuric acid supply pump P is equipped with, for example, two units of one unit (pump PA) for normal use and the other unit (pump PB) for standby for maintenance and inspection. The sulfuric acid supply pump P is configured to include, for example, three or more diaphragms D (DA1 to DA3, DB1 to DB3), and discharge ports S (Sa1 to Sa3, Sb1 to Sb3) are provided for the respective diaphragms D.
As illustrated in FIG. 8, for example, in a case in which the sulfuric acid supply pump P is equipped with the three diaphragms D provided with the discharge ports S (three discharge ports), sulfuric acid is supplied to the sulfuric acid addition pipe LA through two discharge ports, discharge port Sa1 and discharge port Sa2, and to the sulfuric acid addition pipe LB through one discharge port Sa3. Hence, it is possible to add sulfuric acid through the sulfuric acid addition pipe LA in an amount to be about twice the amount of sulfuric acid added through the sulfuric acid addition pipe LB.
As described above, each of the two sulfuric acid supply pumps PA and PB is commonly connected to the sulfuric acid addition pipe LA and the sulfuric acid addition pipe LB. For example, in the case of using the sulfuric acid supply pump PA, valves Va1 and Va2 on the sulfuric acid supply pump PA side are opened while valves Va3 and Va4 on the sulfuric acid supply pump PB side are closed among the valves V provided to the sulfuric acid addition pipe LA. In addition, valves Vb1 and Vb2 on the sulfuric acid supply pump PA side are opened while valves Vb3 and Vb4 on the sulfuric acid supply pump PB side are closed among the valves V provided to the sulfuric acid addition pipe LB. The flow rate of sulfuric acid to be supplied into the sulfuric acid addition pipe L can be adjusted by adjusting the pump speed of the sulfuric acid supply pump P and the stroke of the piston.
Specifically, the pump speed is common to all of the discharge ports (for example, the discharge ports Sa1, Sa2, and Sa3 in the pump PA) provided to the sulfuric acid supply pump P, and the stroke can be individually adjusted at each of the discharge port Sa1, the discharge port Sa2, and the discharge port Sa3, for example, in the case of the pump PA. Incidentally, there is an appropriate stroke for the pump speed, and when the stroke is not appropriately adjusted not only is the flow rate of sulfuric acid added not stabilized but also there is a concern that the pipe L becomes damaged or the like, as well as the pump P and the pipe L vibrate to cause a burden on the facility.
Meanwhile, in the nickel smelting method by the high pressure acid leaching method, the sulfuric acid supply pump P is used under a severe condition that the pump P is continuously operated at a discharge pressure exceeding the internal pressure of the autoclave 1 since the treatment is performed by using the autoclave 1 under high temperature and high pressure conditions. For this reason, the frequency of maintenance and inspection and the frequency of occurrence of problems increase in the sulfuric acid supply pump P. Hence, a plurality of sulfuric acid supply pumps P (“two” in general) is usually prepared so that operation is not stopped even at the time of such maintenance and inspection or the occurrence of problems. By preparing a plurality of sulfuric acid supply pumps P in this manner, in normal operation one sulfuric acid supply pump (for example, the sulfuric acid supply pump PA) is operated and the other sulfuric acid supply pump (for example, the sulfuric acid supply pump PB) stands by. Incidentally, the standby sulfuric acid supply pump P is also called a “spare machine”. Moreover, at the time of maintenance and inspection, it is usual to switch the series of the operating sulfuric acid supply pump PA and the standby sulfuric acid supply pump PB, that is, to put the sulfuric acid supply pump PA in a standby state and the sulfuric acid supply pump PB in a working state and to continue the operation by using the sulfuric acid supply pump PB while performing the maintenance and inspection of the sulfuric acid supply pump PA.
Here, in the case of switching the sulfuric acid supply pump P to the spare machine (for example, the sulfuric acid supply pump PB), it is necessary to start the operation of the sulfuric acid supply pump PB of the spare machine after the sulfuric acid supply pump PA in operation is stopped. Hence, the addition of sulfuric acid to the autoclave 1 is temporarily stopped, and this does not only decrease the working rate but also greatly decreases the leaching rate of nickel, cobalt, and the like, thus it is required to adjust the amount of neutralizer or the like to be added in the steps after the leaching step in the smelting method of nickel oxide ore. Moreover, a series of switching activities including the adjustment of these added amounts takes at least several hours.
As described above, it is necessary to perform the switching activity for the maintenance and inspection while the frequency of maintenance and inspection of the sulfuric acid supply pump P is high since operation is carried out under severe conditions, and it is thus difficult to maintain operation at a high load and a high working rate. Furthermore, it is also required to adjust the amount of the chemicals to be added in the steps after the leaching step in association with the switching for the maintenance and inspection and the like.
For example, Patent Document 1 discloses a method to simplify the smelting process as a whole and to increase the efficiency by simplification of the treatments in the leaching step and the solid-liquid separation step, a decrease in the amount of neutralizer consumed and the amount of sediment in the neutralization step, the efficient and repeated use of water, and the like in the wet smelting method to recover nickel from nickel oxide ore based on high temperature pressure leaching. However, such a prior art has not proposed a sulfuric acid adding facility to be used in the leaching step or the method for operating the same, and a method of adding sulfuric acid at a high working rate has been desired.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2005-350766